Apparatus and method for integral bypass diode in solar cells

ABSTRACT

A solar cell having a multifunction solar cell structure with a bypass diode is disclosed. The bypass diode provides a reverse bias protection for the multijunction solar cell structure. In one embodiment, the multifunction solar cell structure includes a substrate, a bottom cell, a middle cell, a top cell, a bypass diode, a lateral conduction layer, and a shunt. The lateral conduction layer is deposited over the top cell. The bypass diode is deposited over the lateral conduction layer. One side of the shunt is connected to the substrate and another side of the shunt is connected to the lateral conduction layer. In another embodiment, the bypass diode contains an i-layer to enhance the diode performance.

[0001] This application is a continuation-in-part of U.S. application Ser. No. 09/999,598, entitled “An Apparatus and Method for Optimizing the Efficiency of a Bypass Diode in Solar Cells,” filed on Oct. 24, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates to the field of semiconductor devices. More specifically, the present invention relates to the photovoltaic solar cells.

DESCRIPTION OF THE RELATED ART

[0003] Photovoltaic cells, also called solar cells, are one of the most important new energy sources that have become available in the past several years. Considerable effort has gone into solar cell development. As a result, solar cells are currently being used in a number of commercial and consumer-oriented applications. While significant progress has been made in this area, the requirement for solar cells to meet the needs of more sophisticated applications has not kept pace with demand. Applications such as satellites used in mobile and telephone communications have dramatically increased the demand for solar cells with improved power and energy conversion characteristics.

[0004] In satellite and other space related applications, the size, mass, and cost of a satellite power system are dependent on the power and energy conversion efficiency of the solar cells used. Putting it another way, the size of the payload and the availability of on-board services are proportional to the amount of power provided. Thus, as the payloads become more sophisticated, solar cells, which act as the power conversion devices for the on-board power systems, become increasingly more important.

[0005] Solar cells are often used in arrays, an assembly of solar cells connected together in a series. The shape and structure of an array, as well as the number of cells it contains; are determined in part by the desired output voltage and current.

[0006] When solar cells in an array are receiving sunlight or are illuminated, each cell will be forward biased. However, if any of the cells are not illuminated, because of shadowing or damage, those shadowed cells may be forced to become reversed biased in order to carry the current generated by the illuminated cells. This reverse biasing can degrade the cells and can ultimately render the cells inoperable. In order to prevent reverse biasing, a diode structure is often implemented.

[0007] The purpose of the bypass diode is to draw the current away from the shadowed or damaged cell. The bypass becomes forward biased when the shadowed cell becomes reverse biased. Rather than forcing current through the shadowed cell, the diode draws the current away from the shadowed cell and maintains the connection to the next cell.

[0008] A conventional bypass diode is typically connected to the exterior of a solar cell array. A problem associated with this type of bypass diode is that it is difficult to manufacture and also less reliable because the exterior assembly is performed by the array assemblers rather than the cell manufacturer.

[0009] Another conventional method for protecting the solar cell is to place a bypass diode between adjacent cells wherein the anode of the bypass diode is connected to one cell and the cathode of the diode is connected to an adjoining cell. However, a problem associated with this technique is that it complicates the manufacturing process and is more difficult to assemble the solar cell array.

[0010] A third technique for protecting the solar cell involves forming a recess on the solar cell structure and placing a bypass diode in the recess. Because of the fragility of the cells this technique is difficult to implement in a manufacturing line. In addition, the adjoining cells need to be connected to the diode by the array assembler.

[0011] Thus, what is needed is a mechanism and method to enhance the efficiency and performance of bypass diodes in multijunction solar cell structures.

SUMMARY OF THE INVENTION

[0012] A solar device having a multifunction solar cell structure with a bypass diode is disclosed. The bypass diode provides a reverse bias protection for the multijunction solar cell structure. In one embodiment, the multifunction solar cell structure includes a substrate, a bottom cell, a middle cell, a top cell, a bypass diode, a lateral conduction layer, and a shunt. The lateral conduction layer is deposited over the top cell. The bypass diode is deposited over the lateral conduction layer. One side of the shunt is connected to the substrate and another side of the shunt is connected to the lateral conduction layer. In another embodiment, the bypass diode contains an i-layer to enhance the diode performance.

[0013] Additional features and benefits of the present invention will become apparent from the detailed description, figures and claims set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The present invention will be understood more fully from the detailed description given below and from the accompanying drawings of various embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments, but are for explanation and understanding only.

[0015]FIG. 1 illustrates one embodiment of the present invention, a multijunction solar cell, after the completion of all processing steps, illustrating the composition of such embodiment;

[0016]FIG. 2 illustrates the two paths current in the cell illustrated in FIG. 1 can take, given a particular set of circumstances;

[0017]FIG. 3 illustrates one embodiment of the present invention, a multijunction solar cell, prior to any processing steps;

[0018]FIG. 4 illustrates a first processing step used to construct one embodiment of the present invention;

[0019]FIG. 5 illustrates the second and third processing steps used to construct one embodiment of the present invention;

[0020]FIG. 6 is a block diagram illustrating a schematic sectional view showing a multijunction solar cell structure having a bypass diode in accordance with one embodiment of the present invention;

[0021]FIG. 7 is a logic diagram illustrating a triple junction solar cell structure and a bypass diode in accordance with one embodiment of the present invention;

[0022]FIG. 8 is a block diagram illustrating a detailed schematic sectional view showing a triple junction solar cell structure having a bypass diode and a shunt in accordance with one embodiment of the present invention;

[0023]FIG. 9A-9E are block diagrams illustrating a process of manufacturing a multijunction solar cell structure with a bypass diode and a shunt in accordance with one embodiment of the present invention; and

[0024]FIG. 10 is a flow chart illustrating a method of manufacturing a multijunction solar cell structure with a bypass diode in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

[0025] A method and an apparatus of solar cell with multifunction solar cell structure having a bypass diode with an i-layer are described.

[0026] In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that these specific details may not be required to practice the present invention. In other instances, well-known circuits and devices are shown in block diagram form to avoid obscuring the present invention.

[0027] It is understood that the present invention may contain transistor circuits that are readily manufacturable using well-known CMOS (“complementary metal-oxide semiconductor) technology, or other semiconductor manufacturing processes. In addition, the present invention may be implemented with other manufacturing processes for making digital devices.

[0028] The present invention relates to a multijunction solar cell with at least one integral monolithic bypass diode. The layers comprising the solar cell are particularly chosen for their combination of efficiency and manufacturability. As discussed below, one embodiment consists of a multijunction structure with at least three junctions, with a unique modified buffer structure.

[0029] The process of manufacturing the solar cell with an integral monolithic bypass diode is comprised of five distinct steps, which are described below.

[0030]FIG. 1 is an illustration of an embodiment of the invention, a monolithic solar cell with an integral bypass diode. FIG. 2 is a series of schematic drawings of the two possible current paths through the cell.

[0031]FIG. 1 shows a multijunction solar cell 100 with a cell of Indium Gallium Phosphorus (InGaP) 101 and a cell of Gallium Arsenide (GaAs) 102 over a GaAs buffer 103 on top of a Germanium (Ge) substrate 104. When the solar cell 100 is illuminated, both a voltage and a current are generated. FIG. 2A represents the solar cell as seen in FIG. 4, without the metalization 107 and lateral conduction layer 113 described below. If the solar cell is illuminated, there will be no barrier to the current following the cell path 201 through the layers of the solar cell: the Ge junction 104, the GaAs junction 102, and the InGaP junction 101.

[0032] However, when the solar cell 100 is not receiving sunlight, whether because of shading by a movement of the satellite, or as a result of damage to the cell, then resistance exists along the cell path 201. As solar cells exist in an array, current from illuminated cells must pass through shaded cells. If there were no diode, the current would force its way through the cell path 201, reversing the bias of such cells and degrading, if not destroying them.

[0033] If the cell contains a diode, however, the current can be offered an alternate, parallel path 202, and the shaded cells will be preserved. The problem with this concept has been the difficulty in creating a diode that is relatively easy to manufacture and which uses a very low level of voltage to turn on and operate. The invention described herein solves these problems.

[0034] If a cell is shaded or otherwise not receiving sunlight, in order for the current to choose the diode path 202, the turn on voltage for the diode path 202 must be less than the breakdown voltage along the cell path 201. The breakdown voltage along the cell path will typically be at least five volts, if not more. The Schottky contact 403 requires a relatively small amount of voltage to “turn on”-600 milivolts. However, to pass through the Ge junction 104, the bias of the Ge junction 104 must be reversed, requiring a large voltage. Reversing the bias of the Ge junction 104 requires approximately 9.4 volts, so nearly ten volts are needed for the current to follow the diode path 202 in FIG. 2A. Ten volts used to reverse the bias of the Ge junction is ten volts less than otherwise would be available for other applications. The device illustrated by FIG. 4 is therefore a functioning bypass diode, but an inefficient one from a power utilization perspective.

[0035] To address this inefficiency, in the metalization process in which the Titanium Gold (TiAu) contacts 109, 110 are added to the solar cell, an additional layer of metal 107 is added as well. In the embodiment shown in FIG. 1, the metal is TiAu, although practitioners in the art will be well aware that other metals can also be used.

[0036] The effect of the metal 107 is to “short” the Ge junction 104 to the base of the Ge cell 104. Because of the short, a minimal voltage is required to pass current between the layer 113 and the Ge substrate. No longer is a high voltage required to force the current through the Ge junction 104. The current flows easily through the “short path” 107. FIG. 2B provides a schematic representation. If the solar cell is shaded, no longer is the cell forced into reverse bias to pass the current of the array string. There is a much less resistive path, requiring a much lower voltage drop, for the current to pass through the bypass diode 202. With the addition of the metalization 107, the Ge cell 104 is shorted. As a result, rather than a reverse biased diode with a 9.4 turn-on voltage, the current instead encounters an ohmic resistance path represented by the resistor 204.

[0037] The layer is doped to about 7 to 8 times 10¹⁷ cm³ to do two things. First, it reduces the contact resistance of the metal layer 107 and second, it provides a low resistance path for the lateral conduction layer. Without the lateral conduction layer, the resistance at the resistor 204 is approximately 20 ohms. 20 ohms represents a significant drain on the current of the solar cell. To reduce this resistance, a lateral conduction layer 113 is added to the solar cell. FIG. 2C represents the current paths in the solar cell as depicted in FIG. 1. When the solar cell is shaded, the current will flow to the resistor 204. Because of the presence of the lateral conduction layer 113, the resistance at the resistor can be as low as 0.4 ohms.

[0038] The manufacturing process for the solar cell 100 comprises five steps. FIG. 3 shows a multijunction solar cell 100 and the component parts: the multijunction structure 301 and the buffer structure 302. In the illustrated embodiment, a top cell comprised of an n-on-p InGaP₂ 101 is grown over a cell of n-on-p GaAs 102. A third diffused Ge junction 104 is formed due to diffusion of As during the growth of buffer layers 103.

[0039] A buffer exists between the upper junctions in the solar cell and the Ge substrate 104, because the upper junctions are fabricated of III-V material, and the entire cell is grown on a Ge substrate 104. Ge is a group IV element, so it has different lattice parameters than group III-V elements. Lattice matching is generally accepted among those skilled in the art as a way to increase the efficiency of a solar cell, and it follows that lattice mismatching decreases a cell's overall efficiency. To achieve lattice matching, the buffer layer is inserted in the manufacture process; normally it is a thick layer of GaAs grown over the Ge substrate. An InGaP layer lattice matches with a GaAs layer much better than with a Ge layer.

[0040] The buffer structure 302 is comprised of the following: an InGaP Schottky contact 303 as the top layer of the buffer structure 302. This will later form the Schottky diode. The buffer structure 302 is also comprised of an additional etch stop 304. The etch stop 304 enables the device to be more easily manufactured. Upon “wet etching” the etch stop creates barriers during processing which facilitate formation of the bypass diode. The lateral conduction layer 113 exists in this buffer layer, to more efficiently guide the current out of the diode, as discussed above.

[0041] As shown on FIG. 4, the first step in the manufacturing process is to make a “wet etch” that comes down into the cell 100 and terminates at the InGaP layer 403.

[0042] A TiAu contact 110 is formed upon the InGaP layer 403. A TiAu contact 109 is formed at the top of the cell to make an ohmic contact with the n⁺-GaAs layer 112. The TiAu contact 110 on the InGaP layer 403 makes a Schottky contact, which is non-ohmic. In other words, instead of looking like a resistor, such contact 403 and the TiAu contact 110 forms a diode.

[0043] The second step in the manufacturing process is demonstrated in FIG. 5. FIG. 5 shows a “mesa etch” 501 down to the level of the Ge cell 104. The primary purpose of this step is to create a true diode 106, electrically isolating the junctions 105 within the solar cell from the diode 106. When the entire solar cell is manufactured and the metal contacts are bound and the cell is packaged, the cell 105 and the diode 106 will be parallel, yet electrically separate.

[0044] The third step is a “shunt etch” 502, which provides a “shelf” on which the metal 107 in the next step will be laid. To make etches in the middle of manufacturing without etch stops among the layers, one would have to use a “minute etch” which would be extremely difficult to use with any degree of precision in this instance. The etch stop 304 allows the solar cell to be manufactured more efficiently.

[0045] The fourth step is the metalization process. The TiAu contacts 109, 110 are added, and the metal layer 107 is added. In this embodiment the metal comprising the layer is TiAu. Where the TiAu contact 110 meets the InGaP layer 403, a Schottky contact is created.

[0046] At the TiAu contact 109 on top of the cell, the TiAu makes an ohmic contact to n-type GaAs 112. That is an ordinary cell conduction for this type of cell to persons skilled in the art. With the TiAu contact 110 at the InGaP layer 403, a Schottky contact is created. However, because the object is to “short out” the Ge cell 104, the contact was made to the highly doped n+-GaAs cell 113. Layer 113 is also a lateral conduction layer. Upon making the contact to the GaAs layer 113, a resistor is created. The resistance at the resistor 204 was approximately 20 ohms, as illustrated in FIG. 2A. 20 ohms of power dissipation can make the cell too inefficient from a power utilization perspective.

[0047] The etch stop at the GaAs buffer contact 304 alleviates this problem. The GaAs buffer contact 113 is n+ doped at the same level as the GaAs buffer contact 112 at the top of the cell. This creates a cell with the same quality of contact between the TiAu contact 109 and the GaAs contact layer 112 at the top of the cell as the metal contact 107 with the GaAs layer 113.

[0048] Modifying the thicknesses of the various layers in the diode 106 is another way to decrease resistance in the diode 106.

[0049] The lateral conduction layer 113 also alleviates the resistance through the diode 106, from 20 ohms to as low as 0.4 ohms. The current path passes through the diode 106, and the thickness of the diode would ordinarily cause some resistance, but the lateral conduction layer 113 helps the current move to the metal more efficiently. In this embodiment, the lateral conduction layer is made of highly doped n⁺-GaAs. The shunt layer 107 can also be made to partially or completely surround the contact 110, further lowering the series resistance.

[0050] The lateral conduction layer 113 and the metalization 107 are the two most important means to lessen the amount of voltage needed to “turn on” the diode and bypass the shaded cell. By reducing the series resistance, the amount of localized I²R heating is also reduced. The process is also unique because the amount of processing steps are reduced, as the bypass diode layers are grown internally to the buffer layers of the cell, rather than as additional layers that have to be grown on top of the cell. The current device provides for a low bypass diode turn on, as well as a low series resistance bypass diode. Completion of the bypass diode circuit requires a soldered or welded interconnect made between contacts 109 and 110. This can be done as part of the usual interconnect weld.

[0051] The fifth step in the manufacturing process is to apply the anti-reflective coating and include etches where external contacts will be attached.

[0052] As can be seen from the foregoing, the process by which the diode is manufactured is integral to the manufacture of the cell, and does not require additional manufacturing steps or additional layers to be grown on the cell.

[0053] The following illustrates another embodiment of the present invention in which a bypass diode is epitaxially disposed over a multijunction solar cell with an i-layer.

[0054]FIG. 6 is a block diagram 600 illustrating a schematic sectional view showing a multijunction solar cell structure 640 having a bypass diode 620 in accordance with one embodiment of the present invention. Diagram 600 includes a substrate 602, a multijunction solar cell structure 640, a bypass diode 620, a well 650, and a shunt 630. In one embodiment, the substrate 602 is a germanium substrate. The multifunction solar cell structure 640 further includes a top, middle, and bottom subcells. It should be noted that terms solar cells, cells, and subcells will be used interchangeably herein. The multijunction solar cell structure is divided into two portions 642-644, wherein portion 642 includes solar cell(s) for converting solar power to electrical power and portion 644 contains a bypass diode 620.

[0055] Referring to FIG. 6, the multijunction solar cell structure 640 is a multijunction solar cell structure wherein a bottom solar cell 604 is deposited over the substrate and a middle solar cell 606 is deposited over the bottom solar cell 604. The top solar cell 608 of the multifunction solar cell structure is deposited over the middle solar cell 606. Each solar cell within the multijunction solar cell structure is designed to convert the solar energy within a range of wavelength λ of the solar spectrum. For example, the top solar cell 608 of the multijunction solar cell structure is designed to convert the high frequency portion of the solar spectrum into electrical energy. The high frequency portion may include ultraviolet, X-rays, and/or Gamma rays of the solar spectrum. In one embodiment, the high frequency portion covers λ in a range of approximately 700 nm to 100 nm. The middle solar cell 606 is responsible for converting the solar energy in a range of ultraviolet, visible light, and/or portions of infrared of the solar spectrum, which may be approximately between 90 nm to 1000 nm. The bottom solar cell 604 is responsible for converting the solar energy in a range of infrared, microwaves, and/or radio waves, which may be approximately between 700 nm and/or greater.

[0056] It is known to one skilled in the art that the solar spectrum could be divided into more than three regions and each region has an associated solar cell for capturing photons within the respected region. It should be further noted that the underlying concept of the present invention applies to multijunction solar cell structure 640 containing more or less than three subcells.

[0057] Diagram 600 further includes a lateral conduction layer 610 and a stop etch layer 612. For one embodiment, the lateral conduction layer 610 is heavily doped so that it has the property of high electrical conductivity. The stop etch layer 612, in one aspect, is needed to create a shunt contact pad 652 during the etching process.

[0058] Referring to FIG. 6, the bypass diode 620 includes an n-type layer 626, i-type layer 624, and p-type layer 622. For example, n-type layer 626 could be an n-doped gallium arsenic (“GaAs”) layer and a p-type layer 622 could be a p-doped GaAs layer. In one embodiment, a bypass diode with p-on-n polarity is formed when a p-type compound layer is deposited over an n-type compound layer. In another embodiment, a bypass diode with n-on-p polarity may be formed when an n-type compound layer is deposited over a p-type compound layer. The i-type layer 624 is also referred to as an intrinsic layer, lightly doped layer, i-layer and/or non-doping layer. It should be noted that terms i-type layer, intrinsic layer, lightly doped layer, i-layer and undoped layer are interchangeable herein. A function of i-layer 624 is to reduce defect breakdown such as microplasma breakdown. In other words, i-layer 624 reduces leakage current when the bypass diode 620 is in reverse bias mode. As discussed above, the bypass diode 620 preserves the integrity of the solar cell by preventing the solar cell from entering the reverse bias mode.

[0059] In one embodiment, the bypass diode 620 is epitaxially formed over the multijunction solar cell structure 640 so that the bypass diode 620 becomes an integral part of the solar cell structure. In other words, the bypass diode 620 is part of the monolithic solar cell structure. During the manufacturing process, for instance, once an n-type layer 626 is deposited over the stop etch layer 612, an i-type layer 624 is deposited over the n-type layer 626. A bypass diode is completed after a p-type layer 622 is deposited over the i-type layer 624. An advantage of an integral bypass diode 620 is to allow the bypass diode to be manufactured at the same time the multijunction solar cell structure 640 is manufactured. The bypass diode 620 is electrically isolated from the active portion of the solar cell by well 650.

[0060] Well 650, in one embodiment, is created by an etch process, such as a mesa etch. This generates a physical space or gap between the solar cell and the bypass diode 620. In other words, well 650 provides an electrical separation between the active portion of the solar cell and the bypass diode 620. Well 650 also provides a path allowing a shunt 630 to access the substrate. In one embodiment, once shunt 630 is deposited, well 650 may be filled with non-conductive materials, such as anti-reflective materials. It should be noted that the width of the gap or space created by the well 650 between the active portion of the solar cell and bypass diode depends on the semiconductor technology.

[0061] Shunt 630 is deposited via well 650 wherein one side of the shunt 630 is in contact with the substrate and another side of the shunt 630 is in contact with the lateral conduction layer 610. In one embodiment, one side of the shunt 630 is also in contact with a portion of the multijunction solar cell structure 640, which contains the bypass diode 620. In other words, the shunt 630, in this embodiment, shorts a portion of the multijunction solar cell structure that is underneath the bypass diode 620. In this embodiment, the shunt 630 is made of metal to enable it to pass electric current from the substrate to the bypass diode 620 with minimal current loss. An advantage of using shunt 630 is that it reduces the need for external welding jumpers or shorts, which affect the reliability of the solar cell.

[0062]FIG. 7 is a logic diagram 700 illustrating a triple junction solar cell structure and a bypass diode 620 in accordance with one embodiment of the present invention. The logic diagram 700 includes a top cell 608, a middle cell 606, a bottom cell 604, a bypass diode 620, a resistance block 702, and four paths 710-716. In one embodiment, the resistance block 702 includes resistance from the shorted portion of the multijunction solar cell structure that is situated underneath of the bypass diode 620 and the resistance from the shunt 630.

[0063] During normal approach (e.g., solar cells 604-608 are exposed to sunlight, solar light, light, radiation, and/or photons), the solar cells 604-608 are in forward biased. They converts solar energy to electrical energy and pass electric current between the neighboring solar cells connected in series. It should be noted that the terms sunlight, solar light, light, radiation, and/or photons may be used interchangeable herein. In this embodiment, solar cells are organized in a series. While solar cells 604-608 are in forward biased, bypass diode 620 is reverse biased because bypass diode 620 has an opposite polarity from solar cells. Thus, when bypass diode 620 is in reverse bias mode, no electric current passes through the bypass diode 620.

[0064] When electrical current generated from the neighboring solar cells arrives at solar cells 604-608 via path 710, solar cells 604-608 pass total electrical current, which includes the current converted by solar cells 604-608 and the current arriving from neighboring solar cells through path 710, to path 716 via path 712. Path 716 may be connected to another solar cell and/or other electrical devices.

[0065] However, during the situation in which the solar cells 604-608 are in reverse bias mode when, for example, solar cells 604-608 are shadowed, the bypass diode 620 becomes forward biased. In this situation, bypass diode 620 becomes active and passes the current from neighboring solar cells via path 710 to path 716 through path 714. In other words, when the solar cells 604-608 are in reverse bias mode, the bypass diode 620 becomes forward biased and uses path 714 to pass the current from path 710 to path 716.

[0066] It is understood that the underlying concept of the present invention is applicable if additional solar cells and bypass diodes were added in the logic diagram 700.

[0067]FIG. 8 is a block diagram 800 illustrating a detailed schematic sectional view showing a triple junction solar cell structure 640 having a bypass diode 620 in accordance with one embodiment of the present invention. Referring to FIG. 8, the block diagram 800 includes a substrate 602, a triple junction solar cell structure 640, a bypass diode 620, a well 650, and a shunt 630. The triple junction solar cell structure 640 further includes a bottom, middle, and top subcells 604-608. The block diagram 800 also includes contact pads 802-806, wherein contact pad 806 is deposited over a lateral conduction layer 610 and contact pad 804 is deposited over the bypass diode 620.

[0068] In one embodiment, the substrate is a p-type germanium (“Ge”) substrate 602, which is formed over a metal contact pad 802. The bottom cell 604 contains a p-type Ge base layer 810, a n-type Ge emitter layer 812, and a n-type GaAs nucleation layer 814. The base layer 810 is deposited over the substrate 602. The nucleation layer 814 is deposited over the base layer 810, which in one embodiment can be formed through diffusion from an emitter layer 812. After the bottom cell 604 is deposited, a p-type and n-type tunneling junction layers 816, which are also known form a structure sometimes referred to as tunneling diode, are deposited.

[0069] The middle cell 606 further includes a back surface field (“BSF”) layer 820, a p-type GaAs base layer 822, an n-type GaAs emitter layer 824, and an n-type gallium indium phosphide₂ (“GaInP₂”) window layer 826. The base layer 822 is deposited over the BSF layer 820 once the BSF layer 820 is deposited over the tunneling junction layers 816. The window layer 826 is subsequently deposited on the emitter layer 824 after the emitter layer 824 is deposited on the base layer 822. The BSF layer 820 is used to reduce the recombination loss in the middle cell 606. The BSF layer 820 drives minority carriers from a highly doped region near the back surface to minimize the effect of recombination loss. In other words, a BSF layer 820 reduces recombination loss at the backside of the solar cell and thereby reduces the recombination at the emitter region.

[0070] The window layer 826 used in the middle cell 606 also operates to reduce the recombination loss. The window layer 826 also improves the passivation of the cell surface of the underlying junctions. It should be apparent to one skilled in the art that additional layer(s) may be added or deleted in block diagram 800 without departing from the scope of the present invention. Before depositing the top cell 608, p-type and n-type tunneling junction layers 830 are deposited over the middle cell 606.

[0071] The top cell 608, according to this embodiment, includes a p-type indium gallium aluminum phosphide₂ (“InGaAlP₂”) BSF layer 840, a p-type GaInP₂ base layer 842, an n-type GaInP₂ emitter layer 844, and an n-type aluminum indium phosphide₂ (“AlInP₂”) window layer 846. The base layer 842 is deposited on the BSF layer 840 once the BSF layer 840 is deposited over the tunneling junction layers 830. The window layer 846 is subsequently deposited on the emitter layer 844 after the emitter layer 844 is deposited on the base layer 842.

[0072] According to this embodiment, an n-type GaAs cap layer 850 is employed for enhancing better contact with metal materials. The cap layer 850 is deposited over the top cell 608. The lateral conduction layer 610, formed of n-type GaAs, is deposited over the cap layer 850. An n-type GaInP₂ stop etch layer is deposited over the lateral conduction layer 610. After the stop etch layer is deposited, the bypass diode is epitaxially deposited.

[0073] The bypass diode 620 includes an n-type GaAs layer 860, an i-type GaAs layer 862 layer, and a p-type GaAs layer 864. The n-type layer 860 is deposited over the stop etch layer 612. The i-type layer 862 is deposited over the n-type layer 860. The p-type layer 864 is deposited over the i-type layer 862. After layer 864 is deposited, a contact pad 804 is deposited over the bypass diode 620. Once the contact pad 804 is formed, a p-i-n bypass diode with p-on-n polarity is formed over the solar cell. In another embodiment, an n-i-p bypass with n-on-p polarity bypass diode can be also formed over a solar cell structure using similar process described above. It should be apparent to one skilled in the art that the additional layer(s) may be added or deleted in the bypass diode 620 without departing from the scope of the present invention.

[0074] In one embodiment, a metal shunt 630 is deposited via well 650. One side of the shunt 630 is connected to the substrate 602 and another side of the shunt 630 is connected to the lateral conduction layer 610 and a portion of the triple junction cell 644. An anti-reflection coating may be deposited over certain parts of the solar cell to enhance solar cell performance.

[0075] It should be noted that the multijunction solar cell structure could be formed by any combination of group III to V elements listed in the periodic table, wherein the group III includes boron (B), Aluminum (Al), Gallium (Ga), Indium (In), and thallium (Ti). The group IV includes carbon (C), Silicon (Si), Ge, and Tin (Sn). The group V includes nitrogen (N), phosphorus (P), Arsenic (As), antimony (Sb), and bismuth (Bi).

[0076]FIG. 9A-9E are block diagrams illustrating a process of manufacturing a multijunction solar cell structure 640 with a bypass diode 620 and a shunt 630 in accordance with one embodiment of the present invention. FIG. 9A illustrates a triple junction solar cell structure 900 with an integral bypass diode 602 epitaxially formed on the triple junction solar cell structure 900. The triple junction solar cell 900 includes a bottom, middle, and top cell 604-608.

[0077]FIG. 9B illustrates that a portion 922 of bypass diode 602 has been etched away. FIG. 9C illustrates that a well 932 is created through an etching process, such as a mesa etch method. FIG. 9D illustrates that a second portion 942 of the bypass diode 620 is etched away. The stop etch layer 612 is, in one embodiment, used to control the etching process to remove the portion 942 of the bypass diode 620 to create a shunt contact pad 652. FIG. 9E illustrates the next step of formation of the shunt 952. It should be apparent to one skilled in the art that additional layers and steps may be added or deleted without departing from the scope of the present invention.

[0078]FIG. 10 is a flow chart 1000 illustrating a method of manufacturing a multijunction solar cell structure with a bypass diode in accordance with one embodiment of the present invention. At block 1010, the process deposits a germanium substrate. In one embodiment, the germanium substrate is deposited over a contact layer. Once the substrate is formed, the process moves to block 1012.

[0079] At block 1012, the process deposits a solar cell. In one embodiment, the solar cell is a triple junction solar cell, which includes a bottom, middle, and top subcells. Moreover, the bottom subcell may be a germanium solar subcell and the middle subcell may be a GaAs solar subcell. The top subcell may be a GaInP2 solar subcell. It should be noted that it does not depart from the scope of the present invention if the homojunction subcells are replaced with heterojunction subcells. After the solar cell is formed, the process proceeds to block 1014.

[0080] At block 1014, the process deposits a lateral conduction layer over the solar cell. In one embodiment, the lateral conduction layer is a n-doped GaAs layer, which is used as the shunt contact pad. Once the lateral conduction layer is deposited, the process proceeds to block 1016.

[0081] At block 1016, the process deposits a bypass diode over the lateral conduction layer. In one embodiment, after a stop etch layer is deposited on the lateral conduction layer, an n-type GaAs layer is deposited over the stop etch layer. After an i-type GaAs layer is deposited over the n-type layer, a p-type GaAs layer is deposited over the i-type layer. In one embodiment, the concentration of n dopant in the n-type GaAs layer is between 10¹⁷ to 10¹⁸. Like n-type layer, the concentration of p dopant in the p-type GaAs layer is between 10¹⁷ to 10¹⁸. In contrast, the concentration of dopant for i-tape GaAs layer is less than 10¹⁶. Once the bypass diode is formed, the process moves to block 1018.

[0082] At block 1018, a well or gap or space is created to provide electrical separation between the bypass diode and the solar cell. Well also allows the shunt to access the substrate. After the creation of well, the process proceeds to block 1020.

[0083] At block 1020, a shunt is deposited along a portion of the multijunction solar cell structure wherein one side of the shunt is connected to the substrate and another side of the shunt is connected to the shunt contact pad.

[0084] In the foregoing specification the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than restrictive sense 

What is claimed is:
 1. A solar device comprising: a substrate; a multijunction solar cell structure having at least a first, second, and third subcells disposed over the substrate; a lateral conduction layer deposited over at least a portion of the multijunction solar cell structure; and a bypass diode having a p-type, i-type, and n-type layers, deposited over the lateral conduction layer.
 2. The solar device of claim 1, further comprising a well in the multijunction solar cell structure to provide electrical separation between the subcells and the bypass diode.
 3. The solar device of claim 2, further comprising a shunt having a first and second contacting ends, wherein the first contacting end of the shunt is connected the lateral conduction layer and the second contacting end of the shunt is connected to the substrate via the well.
 4. The solar device of claim 1, further comprising a stop etch layer deposited over the lateral conduction layer.
 5. The solar device of claim 1, further comprising a first and second contact layers, wherein the first contact layer is deposited adjacent the bypass diode and the second contact layer is disposed adjacent the substrate.
 6. The solar device of claim 1, wherein the substrate is a germanium (“Ge”) substrate.
 7. The solar device of claim 6, wherein the multijunction solar cell structure is a triple junction solar cell.
 8. The solar device of claim 7, wherein the first subcell is a bottom solar cell, the second subcell is a middle solar cell, and the third subcell is a top solar cell.
 9. The solar device of claim 8, wherein the bottom solar cell further includes: a p-doped Ge base layer deposited over the Ge substrate; an n-doped Ge emitter layer deposited or formed by diffusion over the base layer; and an n-doped nucleation layer deposited over the emitter layer.
 10. The solar device of claim 9, wherein the middle solar cell further includes: a p-doped back surface field (“BSF”) layer deposited over the bottom solar cell; a p-doped gallium arsenic (“GaAs”) base layer deposited over the BSF layer; an n-doped GaAs emitter layer deposited over the base layer; and an n-doped indium gallium phosphide₂ (“InGaP₂”) window layer deposited over the emitter layer.
 11. The solar device of claim 10, wherein the top solar cell further includes: a p-doped indium gallium aluminum phosphide (“InGaAlP”) back surface field (“BSF”) layer deposited over the middle solar cell; a p-doped GaInP₂ base layer deposited over the InGaAlP BSF layer; an n-doped GaInP₂ emitter layer deposited over the InGaP₂ base layer; and an n-doped aluminum indium phosphide₂ (“AlInP₂”) window layer deposited over the GaInP₂ emitter layer.
 12. The solar device of claim 11, further comprising a n-doped GaAs cap layer deposited between the top solar cell and the lateral conduction layer.
 13. The solar device of claim 12, wherein the lateral conduction layer is an n-doped GaAs layer for conducting electrical current.
 14. The solar device of claim 1, wherein the p-type layer of the bypass diode is a p-doped GaAs layer and the n-type layer of the bypass diode is an n-doped GaAs layer.
 15. The solar device of claim 14, wherein the i-type layer is a lightly doped GaAs layer for reducing defect breakdown.
 16. The solar device of claim 14, wherein the i-type layer is an undoped GaAs layer for reducing defect breakdown.
 17. A solar cell structure comprising: at least one solar cell disposed over a germanium (“Ge”) substrate; a lateral conduction layer deposited over a portion of the solar cell structure; a bypass diode deposited over the lateral conduction layer; and a shunt having a first and second contacting sides formed between the solar cell and the bypass diode, wherein the first contacting side of the shunt is connected to the substrate and the second contacting side of the shunt is connected to the lateral conduction layer.
 18. The solar cell structure of claim 17, further comprising a well situated between the solar cell and the bypass diode, wherein the well provides electrical separation between the solar cell and the diode.
 19. The solar cell structure of claim 18, further comprising a stop etch layer deposited over the lateral conduction layer.
 20. The solar cell structure of claim 19, further comprising a first and second contact layers, wherein the first contact layer is deposited over the bypass diode and the second contact layer is disposed over the substrate.
 21. The solar cell structure of claim 17, wherein the solar cell contains a bottom, middle, and top subcells.
 22. The solar cell structure of claim 21, wherein the bottom subcell further includes: a Ge base layer deposited over the substrate; a Ge emitter layer deposited or formed by diffusion over the base layer; and a nucleation layer deposited over the emitter layer.
 23. The solar cell structure of claim 22, wherein the middle subcell further includes: a back surface field (“BSF”) layer deposited over the bottom solar cell; a gallium arsenic (“GaAs”) base layer deposited over the BSF layer; a GaAs emitter layer deposited over the base layer; and an indium gallium phosphide₂ (“InGaP₂”) window layer deposited over the emitter layer.
 24. The solar cell structure of claim 23, wherein the top subcell further includes: an indium gallium aluminum phosphide (“InGaAlP”) back surface field (“BSF”) layer deposited over the middle solar cell; a GaInP₂ base layer deposited over the InGaAlP BSF layer; a GaInP₂ emitter layer deposited over the InGaP₂ base layer; and an aluminum indium phosphide₂ (“AlInP₂”) window layer deposited over the GaInP₂ emitter layer.
 25. The solar cell structure of claim 24, further comprising a GaAs cap layer deposited between the top subcell and the lateral conduction layer.
 26. The solar cell structure of claim 17, wherein the lateral conduction layer is an GaAs layer for transporting electrical current.
 27. The solar cell structure of claim 17, wherein the bypass diode further includes an n-doped GaAs layer deposited over the lateral conduction layer and a p-doped GaAs layer deposited over the n-doped GaAs layer of the bypass diode.
 28. The solar cell structure of claim 17, wherein the bypass diode further includes a p-doped GaAs layer deposited over the lateral conduction layer and an n-doped GaAs layer deposited over the p-doped GaAs layer of the bypass diode.
 29. The solar cell structure of claim 28, wherein the bypass diode further includes an i-type layer, which is a lightly n-doped GaAs layer and deposited between the n-doped GaAs layer of the bypass diode and the p-doped GaAs layer of the bypass diode, for reducing defect breakdown.
 30. The solar cell structure of claim 28, wherein the bypass diode further includes an i-type layer, which is an undoped GaAs layer and deposited between the n-doped GaAs layer of the bypass diode and the p-doped GaAs layer of the bypass diode, for reducing defect breakdown.
 31. A method for manufacturing a solar device comprising: depositing a germanium (“Ge”) substrate; depositing a solar cell having multiple junctions on the Ge substrate; depositing a lateral conduction layer on the solar cell; depositing a bypass diode over the lateral conduction layer; etching a well between the bypass diode and the solar cell; and depositing a shunt between the Ge substrate and the bypass diode through the well, wherein one side of the shunt is connected to the Ge substrate and another side of the shunt is connected to the lateral conduction layer.
 32. The method of claim 31, further comprising depositing a stop etch layer between the lateral conduction layer and the bypass diode.
 33. The method of claim 31, further comprising: depositing a first metal layer on the bypass diode for contact pad; and depositing a second metal layer on the Ge substrate for contact pad.
 34. The method of claim 31, wherein depositing a bypass diode further including: depositing a n-doped gallium arsenic (“GaAs”) base layer over the stop etch layer; depositing a i-doped GaAs layer over the n-doped GaAs base layer; and depositing a p-doped GaAs emitter layer over the i-doped GaAs layer.
 35. The method of claim 31, wherein depositing a solar cell further includes: depositing a Ge-type bottom subcell including a nucleation layer; depositing a GaAs-type middle subcell over the bottom subcell; and depositing a GaInP₂-type top subcell over the middle subcell.
 36. The method of claim 35, further comprising depositing a GaAs cap layer between the top subcell and the lateral conduction layer. 